1. Field
This invention is in the field of paper making, specifically in the treating of alkaline digestion liquor for separating contained lignins and other component solids from the liquid component of the normally waste, spent digestion liquor resulting from the production of paper pulp used in the manufacture of paper.
2. State of the Art
Paper making requires a source of cellulose fibers. Common raw materials as a fiber source are hardwoods and softwoods as well as those of annual vegetable origin, such as wheat and rice straw, bagasse (sugar cane stalks after processing), hemp, and jute. Rag materials, an well as recycled fibers, can also be used. However, wood has been a primary source of cellulose fibers for paper making.
Before use, the wood or other raw material must be processed to release the cellulose fibers. This operation is called xe2x80x9cpulpingxe2x80x9d. At present, commercial pulping operations are of three principal types: mechanical, full chemical, and semichemical. The processes with which the invention is concerned are full chemical and semichemical pulping.
Full chemical and semichemical pulping employ chemical reagents to effect separation of the cellulosic fibers from other components. Wood chips or other raw materials are cooked with suitable chemicals in aqueous solution, usually at elevated temperatures and pressures. The object is to dissolve the organic binders holding the cellulosic fibers, termed xe2x80x9cligninsxe2x80x9d, comprising up to 26% of wood, for example, along with other types of organic molecules, such as saceharide molecules, and other extraneous compounds, leaving the cellulose fibers intact. Though there is some cellulose degradation, the objective can be realized to a commercially satisfactory degree through the use of a variety of chemical reagents. Pulp yields from wood using such processes are usually about 50% of the wood weight.
Lignins have been studied extensively and are said to consist of the noncarbohydrate portion of the cell walls of plant materials. Originally, the lignin content of plant materials was defined as the residue after hydrolysis with strong acid following removal of waxes, tannins, and other extractives, including resins and tall oils. Lignins are amorphous, have high molecular weight, and are predominantly aromatic in structure. In general, the monomeric units comprising lignins can be referred to as p-hydrocycinnamyl alcohols. More specifically, according to The Merck Index, lignins comprise coniferyl, p-coumouryl, and sinapyl alcohols. Their precise composition vary with the method of isolation and with the species, age, growing conditions, etc., of the plant. Lignins are more or less completely removed by chemical pulping, but are essentially not removed at all by mechanical pulping.
Digestion liquors obtained from alkaline pulping usually contain not only all the lignins in the source material, but substantial amounts of cellulose or carbohydrate monomers, other carbohydrates, and, from annual plant materials, such as rice straw, a significant percent by weight of silica. Such used or spent digestion liquors, normally waste, pose problems that are unique in alkaline pulping operations. waste liquor streams from other operations during the paper making process pose different problems, such as removal of the resins and tall oils found in gymnosperm trees.
The lignin solids precipitate when the spent alkaline digestion liquid is acidified and pose a particular problem, because they are polymerized by acidification, producing an amorphous gum (see The Merck Index, p. 864, (1989), S. Budavari, Editor). No easy, inexpensive, or commercially practical method of separating the lignins from the alkali waste liquors has heretofore been known.
Soda and sulfate pulping are both known in the art as being alkaline pulpinq processes. The soda process employs caustic soda (sodium hydroxide), whereas the sulfate process employs sodium sulfide in addition to caustic soda. The sodium sulfide used in the sulfate process results in a stronger cooking liquor and accounts for stronger pulp and faster cooking in the sulfate process as compared with soda pulping. The term xe2x80x9ckraft pulpingxe2x80x9d is an alternative to the term sulfate pulping. For the purposes of this invention, there is no practical difference between the lignin-laden, spent liquors that result from either the sulfate or the soda process or from semichemical or other pulping processes which make use of alkaline agents in conjunction with mechanical means to make pulp, except there is often a relatively great amount of silica in soda process digestion liquor as compared with sulfate (kraft) process digestion liquor. The common link between pulp-making processes with which the invention is concerned is that spent digestion liquors employing alkali, whether buffered or not, become laden with solids and with organic matter, usually referenced to as total organic carbon [TOC], primarily lignins, and that both the inorganic and organic constituents must be recovered or otherwise processed to accommodate environmental concerns as well as to recycle inorganic digestive chemicals. All cooking or pulping reagents employing alkali, especially caustic soda and sodium sulfide, are expensive and the inorganic waste materials are usually too toxic to release spent liquor to the environment.
The sulfate, i.e., kraft, process and the semikraft process are normally used when wood is the raw material. The active pulping ingredients, sodium hydroxide and sodium sulfide, make up an obviously strongly alkaline solution. Standard in the kraft pulping process is the provision of a liquor-recovery cycle in which the organic constituents in the spent digestion liquor (primarily residual lignins and carbohydrates) are burned for steam generation and for recovery of the inorganic, alkaline, pulping chemicals in molten form, they being then solubilized by the addition of water to form so-called xe2x80x9cgreenxe2x80x9d liquor, which is further processed for reuse.
The traditional waste digestion liquor recovery cycle applied most frequently to kraft or semikraft process digestion liquors comprises the step of evaporating digestion waste liquor, the so-called xe2x80x9cblack liquorxe2x80x9d, to a high concentration, i. e., to so-called xe2x80x9cconcentrated black liquorxe2x80x9d or xe2x80x9cblack kraft liquorxe2x80x9d, which is usually up to 70% solids by weight. Organic sulfur compounds are found in the black liquor from the sulfate process in association with sodium sulfide (Na2S). Sodium carbonate (Na2CO2), sodium sulfate (Na2SO4), and silica (SiO2) are also present. Total solids are usually about 15 percent by weight in black liquor after separation from fiber pulp following digestion.
The term xe2x80x9cblack liquorxe2x80x9d is often also applied to other lignin-laden, used or spent digestion liquors, the compositions of which vary with the reagent chemicals used, the raw material, and the particular mill concerned.
The soda process is normally applied to raw materials of annual vegetable origin, such as cereal, e.g. wheat and rice, straw. Such materials normally contain a relatively high percentage of silica, which is solubilized in the digestion liquor. This poses additional separation problems, because, as well known in the art, separating out silica by acidification of the alkaline spent digestion liquor produces a gelatinous or gummy mass that cannot be separated from the liquor in a practical manner. The elevated silica content of liquor derived by pulping such fiber sources, as much as one percent by weight, as compared to the relatively low silica content from wood fiber sources, generally precludes practical application of separation and recovery methods presently known to those skilled in the art.
In the usual kraft recovery process in which silica is a negligible factor, after the black liquor is evaporated to about 70% by weight solids, other procedures, such as vacuum flashing, may be performed to increase even more the preparation of solids for burning. The high-solids-content, kraft black liquor is fed into a reducing recovery furnace provided as part of the usual kraft pulping plant for chemical and energy recovery. The usual reducing recovery furnace requires a large capital investment, and its capacity frequently limits production from a typical kraft pulping plant.
David M. Whalen described a simple method for precipitating lignin from kraft black liquor in Vol. 58, No. 5, May 1975, of the TAPPI Journal, pages 110-112 (see also Whalen et al. U.S. Pat. No. 3,546,200 of Dec. 8, 1970). In that method, kraft black liquor is added slowly and with stirring to a mixture of an organic liquid, such as chloroform, and enough mineral acid to bring the final pH to about 3. The process was successful on a laboratory scale, but the large amounts of organic liquid required made the process impractical on a commercial scale. A more efficient way of separating out the organic constituents, primarily lignins, is still needed.
In making the present invention I have realized that, with a better way of separating out the lignins from the usual digestive liquor, prior art kraft-pulp-producing plants with their high cost recovery furnaces, can still be used in the normal way, but a significant advantage can be achieved, because pulp production from such a plant is not limited by what the recovery furnace can handle. Instead, the excess spent digestion liquor can be processed in accordance with the invention which does not require that the capacity of the standard recovery furnace be increased.
Thus, it was a principal object of the invention to provide a better way of separating out the lignins from the usual spent and waste digestion liquor.
This objective has been accomplished by the addition of a relatively inexpensive, water soluble, surface active, polymeric agent, sometimes referred to hereinafter as xe2x80x9cpolymerxe2x80x9d, to the spent, waste, alkaline digestion liquor before or during the acidification of such digestion liquor and is applicable to the spent, waste digestion liquors from both the commonly used kraft sulfate process and the soda process, whether or not diluted and without the use of heat or pressure.
The polymers used have molecular weights ranging from about five million to about twenty-five million, and may be anionic polymers, such as copolymers of acrylamide and acrylic acid (or sodium acrylate), or partially hydrolyzed polyacrylamide and homopolymers or copolymers of sulfonic acid and acrylamide, which are available as commercial products, such as Percol 919 and Percol 156 from Allied Colloids, Inc., and Nalco 7877 from Nalco Chemicals Company. On the other hand, they may be nonionic polymers based on polyacrylamide chemistry or polyethylene oxides, such as Percol 351, Percol 802, and PEO (polyethylene oxide) available from Allied Colloids, Inc.; or may be cationic polymers of different charge densities, such as Percol 368, Percol 292, and Percol 2802. Nonionic polymers are preferred.
The polymer or a mixture of polymers is added in an amount to bring concentration thereof towithin the range of about 0.05% to about 1.0% by weight in the liquor. To achieve such percentage range, the polymer or mixture of polymers is added in amount from about 0.1 to about 5.0 pounds/ton of dry organic material in the digestion liquor. The preferred level of addition is about one pound per ton.
As the acidification of such digestion liquors can lead to the generation of gases, depending on the alkaline salt used in the pulping process, it is preferred to add a different water soluble, surface active agent, or combination of agents, of low molecules weight, such as a fatty acid or soap of fatty acid, a polysilicone, or a succinate, which has a carbon chain containing from about eight carbon atoms to about eighteen carbon atoms, primarily to enhance the action of the polymer, but, incidentally, to control foaming. It has been found that the presence of such an agent in the range of from about 0.01 to about 1.0 pound per ton of the spent digestion liquor treated has a significant beneficial effect in separation of solids or near solids from the liquid component of the digestion liquor. The preferred level of addition is in the low range of about 0.1 pound per ton of the liquor. The result is the recovery of a substantially clear or very low colored liquid component of the original digestion liquor, which xe2x80x9cclarifiedxe2x80x9d liquid is essentially free of dissolved, higher molecular weight, organic solids (primarily lignins and dissolved carbohydrates). Total organic carbon (TOC) in the clarified liquid may be as low as about 0.01% by weight.
Addition of the indicated other surface active, or defoaming, agent to the digestion liquor prior to acidification produces superior results. A clarified liquid component having a residual total organic carbon content on the order of about 0.02 to about 0.01% by weight can easily be achieved. This residual TOC represents a small amount of simple carbohydrate and a very, very small amount of residual lignins if any.
After the water soluble, surface active, polymer and the additional surface active agent, or defoamer, are in place, the digestion liquor is acidified to a pH below 7, preferably to at least about 3. The lignins agglomerate and at least tend to float to the top of the liquor as a non-gelatinous, non-gummy, solid fraction. The higher the solids concentration of the digestion liquor, the greater the tendency of the coagulant to float or to actually float at the surface of the liquor. There, the lignins can be easily separated by mechanical, gravity separation, as by screening the upper portion of the liquor or by filtering. In a spent digestion liquor not as concentrated, where the solids concentration is less than about 15% by weight, the lignins still coagulate, i.e., they form a non-gelatinous, non-gummy solid fraction, but neither fall out of the liquor nor actually float on top. In this condition, i.e., tending to float, they can still be separated by a mechanical, gravity separation step, such as filtration.
The removed solids are washed for removal of residual salts and are thereafter dewatered and dried. At no point does the process result in a slimy gelatinous mass or amorphous gummy fraction of acid lignins or of acidified silica, as has previously occurred upon acidification of black liquor of either the kraft or the soda process. Moreover, by the process of this invention, a significant advantage is achieved by using the aforedescribed procedure as an alternate method of treating excess digestion liquor produced by the usual kraft pulping plant when operated at over the design capacity of the recovery furnace for handling the residual solids to derive energy and recover chemicals therefrom. Instead of requiring the expenditure of large sums to expand the capacity of the recovery furnace of an existing kraft processing plant, excess production of waste digestion liquor can be handled by the present process and by use of a relatively inexpensive supplemental furnace, such as a fluidized bed furnace, as an economically acceptable capital investment. After separation of the solid fraction from the liquid fraction of the diverted waste digestion liquor, the clear residual liquid component is returned to the flow stream entering the plant. This means that the usual recovery furnace of the kraft pulp processing plant need not be the limiting factor in pulp production for existing sulfate (kraft) processing plants.
In the soda process, inorganic chemicals dissolved in waste digestion liquor would normally not be further processed. However, lignins with silica dissolved in the waste liquor are separated together to produce a solid product, having value, and a waste liquid that can be safely returned to the environment.